Creating function which generates random number between two arguments C

Question

How can i create function in C , which would generate random number }(type double) between set arguments? In lot of languagues its simply just

function randomnumber(min, max) {
  return Math.random() * (max - min + 1) + min;
}

but i fail to find any good solution for type double in c.

... you're using `floor` so you really want a random *integer*? — hobbs, Nov 06 '15 at 16:37
@ameyCU Proposed duplicate deals with `float`, yet the post request `double`. That extra precision/range poses additional concerns that a good answer would provide. — chux - Reinstate Monica, Nov 06 '15 at 16:59
trolkura what type of distribution do you want: linear between `max` and `min`? It is easier to not include one of the limits so like `[min...max)` rather than `[min... max]`. Do you want to be able to generate every `double` between `min` and `max` or just most of them sufficient? — chux - Reinstate Monica, Nov 06 '15 at 17:04

score 1 · Accepted Answer · edited Nov 06 '15 at 21:30

1

There are several methods. Answer 1 above is ok for min, max as doubles. The important thing to remember for rand() is that it returns a random integer between 0 and RAND_MAX inclusive. RAND_MAX is a least 2^15, but on most systems 2^31. If min and max are integers and their difference is less than RAND_MAX then a very easy method is:

delta = max - min;
r  = rand() % delta + min;

edited Nov 06 '15 at 21:30

David C. Rankin

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answered Nov 06 '15 at 17:06

akm27

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score 0 · Answer 2 · answered Nov 06 '15 at 16:54

0

#include <stdlib.h> /* rand(), srand(), RAND_MAX */

double randd(double min, double max) {
    int r = rand();
    double delta = max - min; /* assume max > min */
    double tmp = r / (RAND_MAX + 1.0); /* generate up to but excluding max */
    return tmp * delta + min;
}

answered Nov 06 '15 at 16:54

pmg

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See it running: http://ideone.com/EOGwIJ – pmg Nov 06 '15 at 17:04
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OP was not very specific on the goals, yet this answer will likely not generate every `double` between `min` and `max`. – chux - Reinstate Monica Nov 06 '15 at 17:06
@chux Depending on `min` and `max` this answer, with its bias, can generate every `double` between those values .... and repeat some of the answers to boot (think 52 bits for the mantissa and 63 bits for the PRNG). – pmg Nov 07 '15 at 15:41
Rare machines have a `int` of 63+ bits - the return type of `rand()` - more likely to be 32-bit (or even 16) and so insufficient to generate all the range of `double` with many `min`, `max`. Had this answers referenced a 63 bit PRNG or express how to compensate for insufficient `RAND_MAX` range, I would have not commented so. – chux - Reinstate Monica Nov 08 '15 at 05:18

Creating function which generates random number between two arguments C

2 Answers2